The present invention relates to a system for improving a characteristics of an output signal of a TV camera in such a way that a vertical resolution thereof on a screen of a TV receiver the output signal can be improved.
The NTSC color TV system has been used in, particular, U.S.A. and Japan, and various proposals have been made on a transmitter and a receiver to obtain a higher quality of image.
It has been known that, in order to compensate for vertical resolution on a TV receiver, a signal processing circuit is provided in a transmitter side for processing an output signal of a TV camera so that a processed video signal is transmitted. An example of such signal processing circuit is one called as a detail correction circuit a construction of which is shown in FIG. 4. In FIG. 4, the detail correction circuit connected to a TV camera 1 includes a matrix circuit 2 having inputs connected to red, green and blue color outputs of the TV camera 1 and adapted to perform a matrix operation of these color signals, series connected delay lines 5-1 and 5-2 each providing one horizontal scan time (H) delay, the delay line 5-1 being adapted to receive luminance signal (Y) on a line 3, multipliers 6-1, 6-2 and 6-3 for multiplying signals at an input of the delay line 5-1, at a junction between the delay lines 5-1 and 5-2 and at an output of the delay line 5-2 with respective coefficients to be described later, respectively, an adder 7 for adding outputs of the multipliers 6-1, 6-2 and 6-3 and a matrix circuit 8 which is an opposite circuit to the matrix circuit 2 and receives I and Q signals on lines 4-1 and 4-2 and an output of the adder 8. An output of this detail correction circuit is shown by a numeral 9 as which an output signal of the TV camera 1 compensated for vertical resolution is provided.
It is known that the resolution of the TV camera is checked by means of the so-called inmegachart on which a test pattern is given. The test pattern includes a number of fine parallel black lines extending vertically on a white background. For the check of horizontal resolution, the test pattern is scanned horizontally in normal direction to the vertical black lines. Then the inmegachart is turned by 90.degree. and the vertical resolution is checked by moving the camera vertically within a limited distance. Since the number of scan lines is 525 in the NTSC system, a monochromatic pattern obtained by scanning this chart includes 262,5 (525/2) periods per image screen. Therefore, it corresponds to the number of periods within a vertical height (H) of the image screen and is usually represented CPH (Cycle Per Height) unit. The value of (525/2)CPH means a most severe condition to obtain such monochromatic image and, since a repetition rate of horizontal scan lines is 15.75 KHz, a multiplied value is usually referred to as vertical frequency. In checking the amplitude characteristics in a direction normal to the camera movement, O CPH is made correspondent to O dB. Thus, a monotoneously decreasing characteristics curve a in FIG. 5 is obtained. Since, in obtaining a video signal from the camera, one field is constituted with 262.5 scan lines which is a half of 525 lines and alternative lines are scanned in a next field, the vertical amplitude characteristics, when only one field is concerned, in a range higher than (525/4) CPH is not enough to express the signal an an image. Therefore, the characteristics curve a is compensated such that a characteristics curve b is obtained. That is, the signals RGB from the camera 1 are converted by the matrix circuit 2 into the Y, I and Q signals and a compensation is performed for the Y signal among them. Since band widths of the I and Q signals are narrow, compensation for them is not performed. The compensation for the luminance signal Y is to provide a peak around (525/4)CPH as shown by the curve b in FIG. 5. In order to realize this compensation, the original signal is supplied to the multiplier 6-1, the original signal delayed by one horizontal scan time (H) is supplied to the multiplier 6-2 and the original signal delayed by two horizontal scan time (2H) is supplied to the multiplier 6-3. A multyiplying coefficient a1 of the multiplier 6-2 is selected as being larger than 1 and a mulitplying coefficient a2 of the multipliers 6-1 and 6-3 is selected as being equal to or smaller than 0 so that the following equation is established. EQU a1+2a2=1.0 (1)
The equation (1) means that a d.c. signal level is constant (O dB at frequency 0). Since a current scan line is amplified by 1 or more and a preceding and subsequent scan lines are inverted in phase and added, a characteristics curve which is symmetrical about (525/4)CPH is obtained. The coefficients necessary to provide the curve b in FIG. 5 are a1=1.2 and a2=-0.1.
Since this compensation for the signal Y is performed with respect to the characteristics signal of only the camera, the curves a and b in FIG. 5 are multiplied with each other, resulting in a curve c in the same figure, which has a peak around [(525/4).times.(2/3)]CPH.
However, it has been found that the characteristics shown by the curve c is not enough to practically improve the resolution. Particularly, it is insufficient for an image portion containing signal components of high frequency band.